1. Field of the Invention
The present invention is in the field of loud-speakers, and more specifically is concerned with a class of loudspeakers that exploits the advantages inherent in the use of solenoids having a length-to-diameter ratio that is considerably greater than in conventional voice coils.
2. The Prior Art
By far, the most common loudspeaker in use today is the permanent magnet loudspeaker, an example of which is shown in FIGS. 1-3. In that type of loudspeaker, the voice coil 8 typically consists of a number of turns of very fine wire wound on a former 9. The length of the winding is usually only a small fraction of its diameter. The length-to-diameter ratio is typically in the range from 0.1 to 0.5.
In the typical prior art loudspeaker shown in FIGS. 1-3, the voice coil 8 fits into a gap between a first pole piece 3 and a second pole piece 4. The former 9 is supported by the diaphragm 10 and the spider 13 keeps the voice coil 8 centered within the gap. The diaphragm 10 is flexibly supported by a flexible membrane 12.
In the arrangement of FIGS. 1-3, the magnetic lines of flux extend radially outwardly from the pole piece 3.
The turns of wire that make up the voice coil 8 extend circumferentially around the gap between the pole pieces 3 and 4.
When a current flows through the voice coil 8, the windings experience a force acting perpendicular to both the flux lines and the direction of current flow, and therefore the force is in the axial direction. The sense of the force depends on the sense of the current flow through the coil 8.
Loudspeakers of the type shown in FIGS. 1-3 are typically designed to minimize the mass of the moving voice coil assembly, and this tends to result in the use of voice coils having relatively low length-to-diameter ratios.
Also, in order to maximize the density of the flux lines within the gap, and hence the strength of the magnetic field there, the axial extent of the gap is often relatively small.
Designers typically have been much concerned with maintaining a uniform magnetic field within the gap.
The design orientation of the prior art leads to several problems which limit the performance of prior art loudspeakers.
The smaller dimensions used in the gap necessitate very accurate positioning of the parts, which necessitates tighter manufacturing tolerances, and this results in higher rejection rates and greater manufacturing costs.
A second problem that results from the design orientation of the prior art is related to the close tolerances required. When relatively high currents are forced through the voice coil, the voice coil may experience ohmic heating to such a degree that it expands radially sufficiently to touch the outer pole face, thereby resulting in undesirable sound effects.
Yet another problem of the prior art approach is that the use of a very strong magnetic field in the gap results in comparatively high motion-induced voltages in the voice coil. According to Lenz's Law this motion-induced voltage is in such a direction as to oppose the motion of the voice coil. Motion-induced voltages are undesirable because they complicate amplifier design.